The invention relates generally to a carriage system for a probe for the diagnosis and monitoring of the operation of an electrical apparatus.
In the field of generating electricity on a commercial scale it is important that elements of the power generating system remain fully functional over their expected working life so that unexpected downtimes and/or catastrophic failures can be avoided. To avoid such problems it is important that elements such as the large stators, which form part of the above-mentioned generating systems are carefully inspected and tested before being sold, after installation at customer site, and during regular periodic maintenance.
The stator core 30 of electric machines (such as schematically depicted in FIG. 1) utilizes thin insulated steel laminations 32 (FIG. 2) to reduce the eddy current flow for higher efficiency operation. The laminations 32 are, as shown in FIG. 2, stacked vertically by placing a dovetail groove 34 of the laminations in the dovetail of a key bar 36, which is attached to a frame of the stator core 30. To hold the laminations together and to prevent lamination vibration, the stator core 30 is axially clamped with a force of about 300–350 psi.
Shorting of the laminations 32 can be caused by manufacturing defects, damage during assembly/inspection/rewind, stator-rotor contact, vibration of loose coil wedges/laminations, foreign magnetic material, etc. If the laminations 32 are shorted for any reason, a larger circulating current is induced in the fault loop that consists of fault-laminations-key bar (see FIG. 2). The typical fault locations 39 are shown in FIG. 3. The circulating fault current 26 increases with the number of shorted laminations and the conductivity between the laminations and the short/key bar. The fault current 26 increases the power dissipation in the stator core and causes localized heating. The hot spots can progress to more severe localized heating and eventually cause burning or melting of the laminations. As a result, the stator bar insulation and windings can also be damaged causing ground current flow through the stator core. Therefore, inter-laminar core faults should be detected and repaired to prevent further damage and to improve the reliability of generator operation.
Various tests have been developed in order to detect imperfections in the stator core 30. The “ring test” relies upon the detection of the eddy current heating caused by the short circuit currents. The stator core 30 is wound with a number of turns (typically less than 10) of electrical cable to form toroidal shaped excitation windings 31 in the manner schematically depicted in FIG. 1. The current level in the windings is chosen such that the flux driven in the stator core 30 is near normal operating levels (approximately 1–1.5 Tesla). The excitation requirement measures several million voltages-amperes (MVA), since several hundred amperes and volts in the coil are needed to achieve the desired flux. The stator core 30 is excited in this manner for several hours. Thermal imaging cameras are used to find “hot spots” on the inner stator surface. These hot spots indicate the location and severity of the inter-lamination short circuits.
However, short circuits that are located below the surface of the stator teeth 37 and slots are difficult to find, since thermal diffusion causes the surface temperature rise to be diffuse/spread out. Because of the high power levels used in the ring test, personnel cannot enter the bore of the stator core 30 during testing. Further, cables used in the test must be appropriately sized for the required MVA level, which leads to long setup and removal times.
The high flux used in the ring test is a concern because: the high currents (e.g., hundreds of amperes and several thousand volts) needed require a test supply capable of several MVA. Also, the high current and voltage levels require care in the selection and installation of the excitation winding on the generator core because they can obscure parts of the core. Furthermore, because the heating test is run on a core that is deprived of its normal cooling system, excessive heating can lead to core damage. The high current and voltage levels impact operator safety, and as mentioned above, personnel are not allowed to enter the core interior when a ring test is running.
To overcome the shortcomings of the ring test, the “EL CID” (Electromagnetic Core Imperfection Detection) test was developed. This test relies upon detection of the magnetic field caused by the short circuit currents that flow due to inter-lamination short circuits. As in the ring test, the generator core is wound with a number of turns in the manner of a toroid. The current level in the windings is chosen such that the core operates at approximately 4% of the normal operating flux. This corresponds to about a 5 volt/meter electric field induced along the core surface. The current requirement is in the 10–30 ampere range, so that a smaller power supply of several kVA can be used. A magnetic potentiometer, referred to as a Chattock coil 38 after its inventor, is used to sense the magnetic fields produced between two adjacent teeth by the short circuit currents that are induced in the inter-lamination insulation faults.
The Chattock coil 38 (also known as the Maxwell worm or magnetic potentiometer) is used to sense the phase quadrature component of the magnetic field produced by any induced inter-laminar currents. Chattock coil voltages equivalent to those produced by a 100 mA or larger test current are used as the indicator for a severe inter-laminar short for the 4% flux excitation level.
The Chattock coil 38 typically spans the width of two adjacent teeth 37 in the manner shown in FIGS. 4 and 5 and is moved along the surface of the stator either by hand or by a robotic carriage. Because the short circuit current path is largely resistive, the magnetic flux created by the short circuit is in phase quadrature with the exciting flux. The signal from the Chattock coil 38 is combined with a reference signal derived from the excitation current so that phase sensitive detection methods can be used to extract the fault signal from the background noise.
A fully digital EL CID system has been developed. This system exhibits improved noise suppression over the previous analog arrangements. Nevertheless, there are a number of anomalies and distortions, which can arise when performing the EL CID test, and these must be interpreted using knowledge and experience of core construction.
The EL CID test involves exciting the core in a manner similar to that of the ring test, but uses much lower voltage and current levels. A flux of 4–5% is normal. The EL CID test procedure exhibits the following characteristics. The current required for this flux can be obtained from a variable transformer that is supplied from a standard electrical outlet. The induced voltage from this low flux is kept to about 5 volts/meter, so personnel can enter the core during the EL CID test to make observations. The induced currents at this flux are low enough not to cause excessive heating, so additional core damage due to testing is not a concern.
The EL CID test is better able to find inter-laminar faults, which are located below the surface. This is a significant advantage over the ring test that relies upon thermal diffusion from the interior hot spot in order to provide detection. However, the EL CID test can exhibit high noise levels, especially when scanning in the end step region 35 (see FIG. 12). The high noise levels are due to the Chattock coil 38 being located on one side of the EL CID trolley, requiring the trolley to be flipped or carefully positioned at the end step region 35. Additionally, handling the trolley occasionally results in breakage of the fine wire that is wound around the Chattock coil 38.
Thus, it is desirable to develop a probe that is not subject to breakage due to handling and which will also not require flipping in the end step region 35.